What is a subdomain?A sub-domain is sort of a "new domain" but under and existing standard domain. Standard domains can is like "www.xyz.com" or "http://xyz.com" whereas sub-domain tends to take the form of "http://subdomain.xyz.com" NOTICE that the "WWW" which stands for World Wide Web is not present in sub-domain.

Will the rankings of standard domain decrease if I use a subdomain?The answer is “NO”. Subdomains are just the same as normal domains in getting rankings. The same things that you do to a standard domain to optimize it for the Search Engines, you will need to apply to a sub-domain to get the desired effect.

What are the advantages of Sub Domains?There are quite a few advantages of using sub domains. One is that you can submit these to directories, and gain independent link's etc, which you could not do with a page such as www.xyz.com/folder/ . Also, all links from a subdomain to the main page are seen as 1 way inbound links, rather than internal links, which is also a big plus point.

What are the disadvantages of Sub Domains?There are few disadvantages of using sub domains. You would find starting a website completely from scratch. This includes things such as battling the Google Sandbox, getting inbound links, and getting the page popular so that it’s crawled, and then ultimately crawled more often.

If subdomain is banned, will it affect main domain?The answer is “YES but not always. This highly depends on the linking between the sub domain and the main domain, close relationship between the two is more likely to leave an effect on the main domain.